Object Using name of prototype function name instead of its property

Question

I created two function A and B. Made A as prototype of B. jsfiddle

function A(){

}

function  B(){
    this.name ="Class B";
}

B.prototype = A;
var b = new B();
alert(b.name); // expected Class B got A

I know I should have used B.prototype = new A();

Is above behavior expected? Is it possible to get "Class B" instead of "A" in above scenario?

Note: I am using above because A has some property associated with it as A.text = "Text" A.text1 = "Text2" and I want these property available to B. so B.prototype == A.prototype. can not be used.

B.prototype = A; is assigning the function A to the prototype of B, that's probably not what you want. — elclanrs
– elclanrs, Commented Nov 22, 2012 at 22:38
That scenario seems irrelevant since you'd probably never want to do that, but yes, you can get "Class B" in implementations that don't support the non-standard .name property on functions. — I Hate Lazy
– I Hate Lazy, Commented Nov 22, 2012 at 22:40
...though it does seem odd that you couldn't assign a property since this is the object, not the prototype. — I Hate Lazy
– I Hate Lazy, Commented Nov 22, 2012 at 22:42

Community · Accepted Answer · 2017-05-23 12:18:43Z

2

Have a look at the possible duplicate What is the reason to use the 'new' keyword at Derived.prototype = new Base and also at this answer on why there are no "classes" in javascript.

Yes, the behaviour is expected. B.prototype, the object from which your b instance inherits, is the function object A. So, it does inherit its name property which is the string "A".

And, this property is immutable (its property descriptor is {configurable: false, enumerable: false, value: "B", writable: false}), so when you try to assign a new value to b.name, this will check for [[CanPut]] which returns false - and nothing happens. in strict mode, an Invalid assignment in strict mode TypeError would be thrown (demo).

You only could overwrite it using Object.defineProperty.

@Edit: I'm not sure why you want those properties inherited by B. They are "static attributes" of the A constructor function and they should stay there. There is currently no way to let a Function inherit from anything else than Function.prototype, so either you go with copying them or your don't use them. Tell us for what you'd need them, and we can find a solution.

Changing the prototype property of functions (and objects in general) does not make them happen to inherit from anything else. The prototype property of functions points to the object from which newly created (constructed) instances will inherit.

edited May 23, 2017 at 12:18

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answered Nov 22, 2012 at 22:37

Bergi

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6 Comments

Pointy Over a year ago

Ah - and that property is not mutable. Durr. I've been scratching my head for a couple of minutes now.

Šime Vidas Over a year ago

The point is that the inherited "name" property is non-writable, so the attempt to create an own property with that name fails.

Anoop Over a year ago

I just wanted to know why prototype property is given more preference than own property?

Šime Vidas Over a year ago

@Sushil Well, read my comments in the other answers. If the inherited property is non-writable, you cannot shadow it by simple assignment. The "name" property of the A function is non-writable, so this.name = ... inside B fails.

Pointy Over a year ago

@Sushil the deal is this: when you mention name in an expression, the runtime first has to determine whether name is a symbol that's already defined. In this case, it is defined, and it's defined as a non-mutable property of the prototype. Thus, the assignment statement does nothing at all.

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Šime Vidas · Accepted Answer · 2012-11-22 22:36:38Z

0

If you want B to "inherit" from A, write:

B.prototype = Object.create( A.prototype );

answered Nov 22, 2012 at 22:36

Šime Vidas

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5 Comments

Pointy Over a year ago

Do you understand why the code as written behaves the way it does? I'm confused by it; maybe I'm just tired :-)

Bergi Over a year ago

Explain why that's better than new A, what the drawbacks are and I'll give you an upvote :-)

Šime Vidas Over a year ago

@Pointy It appears that it's not possible to shadow an inherited "name" property, if it belongs to a function: jsfiddle.net/hG64d/1 I'm not sure why, I'll have to investigate.

sroes Over a year ago

jsperf.com/object-create-vs-constructor-vs-object-literal/7 indicates that using new is faster than object.create

Šime Vidas Over a year ago

@SanderRoesink Those options are not the same. The first two create an object which inherits from Object.prototype, whereas the last one creates an object which inherits from Obj.prototype, which in turn inherits from Object.prototype.

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